Cranes of the above type include essentially an undercarriage--frequently developed for travel--a superstructure rotatable thereon around a vertical axis, as well as at least one boom which is swingable in the vertical plane and on the upper end of which further booms can be pivoted. For the raising--as well as the lowering--of the main boom various devices are known and furthermore necessary, as can be noted, for instance, from Federal Republic of Germany Patent 29 33 382. While the measures disclosed in that patent cover the requirements as to the stability of the crane upon use and its supporting capacity, these measures not only represent a considerable structural expense and thus also considerable weight for the machine also an expenditure of time upon the setting up and removal of the apparatus, which reduces the use-time.
In addition, under the law governing mobile cranes on highways the permissible axle loads are subject to an upper limit. These provisions lead, particularly in the case of large mobile cranes, to a large number of axles, so that 10-axle machines are no rarity. This also means expensive constructions of the undercarriage, in which connection extremely powerful drive units are necessary for the transporting of the entire vehicle. This drive must be transmitted via an expensive longitudinal differential to a plurality of axles which are at a great distance from each other. Due to its high power, the drive of the undercarriage cannot be readily used also for the operation of the crane, so that an additional drive unit with the customary peripheries for the crane is provided in the superstructure. To this end, suggestions are made in Federal Republic of Germany OS 36 39 709 for reducing the cost of manufacture.
None of these previously known proposals, however, has been able in any way to change the fact that the requirement of the highest possible supporting capacities could be satisfied only at a correspondingly high structural expense--with the result of considerably greater weights--and with the disadvantage of costly setup times.